In applications such as fiber optic communication systems, a common type of amplifier is the so-called "trans-impedance amplifier". Such amplifiers have a relatively wideband frequency response and low noise. Stable gain for the amplifier is another desired characteristic. One stage of a trans-impedance amplifier is typically implemented by MOSFET technology using two field-effect transistors (FETs) of the same type. Several stages of this type are connected in cascade to complete the amplifier.
In a single stage of the amplifier, the input signal is applied to the input (gate) electrode of one FET whose output (drain) is connected to the input (gate) of the second FET. The first and second FETs are both operated in the common source mode, that is, their sources are connected to ground. The drain electrodes of the two transistors are connected together and are also connected to a drain voltage source. The amplified output signal is taken from the connected drain electrodes of the two FETS.
It is desirable for the trans-impedance amplifier gain to be constant when operating under varying ambient conditions. The gain depends on such factors as the physical characteristics of the transistors, the input current and the operating conditions. Any variation of the physical characteristics of one FET relative to another in an amplifier stage affects the stability of the overall amplifier gain. Also, the gain of an amplifier stage is sensitive to offsets of the input voltage. That is, if the input voltage has a quiescent voltage other than OV, then the amplifier will operate on portions of the gain curve other than where it was designed to operate and the gain will vary. Another disadvantage is that it is somewhat difficult to design such an amplifier to have a desired gain characteristic.